The performance of many commercial and military electronic systems, including digital, power and microwave systems, is constrained by the cooling capabilities of conventional thermal management systems and methods. Heat flux within devices continues to increase as new technologies are developed. For example, enhanced performance capabilities of digital processors and associated memory devices are accompanied by increased heat generation. Likewise, power devices such as amplifiers, regulators and power converters continue to provide increased capability despite smaller packaging, resulting in higher dissipated power densities.
In the microwave field, the introduction of Gallium Nitride (GaN) transistors in place of traditional gallium arsenide (GaAs) devices has resulted in heat flux increases of at least one order of magnitude. For example, field effect transistors (FETs) produced in GaN can dissipate about 10 watts per millimeter (W/mm) of gate width compared to 1 W/mm in GaAs FETs. Microwave amplifiers, often embodied as monolithic microwave integrated circuits (MMICs), are built by grouping these FETs side by side and in multiple stages to realize a desired or target gain and RF power of a given device. The resulting power density in these MMIC amplifiers has increased from approximately 100 watts per square centimeter (W/cm2) in GaAs applications, to approximately 30K W/cm2 in GaN configurations. In addition, GaN devices can withstand higher temperatures compared to GaAs devices, and thus are operated at higher drive levels which leads to even higher power densities. These high power GaN microwave amplifiers are used in applications ranging from cellular systems (e.g. cellular phones) to radars, target illuminators, communication devices and electronic warfare equipment for the defense industry. The performance of these devices has become limited by the abilities of conventional cooling techniques to efficiently remove the increased heat generated by these systems.
Referring generally to FIG. 1, there is shown a typical “remote cooling” thermal management system 10 for use with small-scale electronics, including integrated circuit packages. Heat generated by a source, embodied herein as a circuit die 12, is transferred through multiple intermediate material layers arranged between die 12 and a heat sink, such as a liquid-cooled cold plate 19. These layers may include, for example, a solder layer 13, a heat sink or heat spreading layer 14, a first epoxy layer 15, a module package including ceramic layer 16 (e.g. a transmitter/receiver module package), and a second epoxy layer 17 for attaching the module package to cold plate 19. As illustrated, each material presents a thermal resistance (Rth) to heat flow and adds to the overall thermal rise from the coolant to the device junction. While such assemblies provide some level of effectiveness, increases in device power and density resulting from packaging multiple devices together, have created systems with even greater cooling requirements. These systems, in many cases, exceed the capability of the present remote cooling approaches to maintain acceptable junction temperatures.
Alternate thermal management systems and techniques are needed.